It is known to detect or record bioelectrical signals, for example, of a human brain, by means of a medicated implant. For detection of bioelectrical signals at least two electrical contacts or electrodes are necessary which are connected to the biological organism or which are coupled to the biological organism. With one of the two contacts, the bioelectrical signal is detected in form of an electrical potential change. A detection of an electrical potential change also is called “derivation of a signal”. The other one of the two contacts is coupled to a static, if possible non-changing, electrical potential within the same organism such that this contact provides a so-called reference potential. The reference potential between the two electrical contacts or the change of this potential difference forms the bioelectrical signal to be detected. This type of detection or derivation is called monopolar derivation. The monopolar derivation is used if the bioelectrical signal is only to be detected at a single contact.
As alternative for the monopolar derivation, also the so-called bipolar derivation is known. With respect to the bipolar derivation, the reference electrode which provides a reference potential for the monopolar derivation, may also be arranged in the electrically active area of the biological organism. As an alternative to this, it is known to arrange the reference electrode as arranged for the monopolar derivation in the electrically passive area of the biological organism, and to provide a third electrical contact which is arranged in the electrically active area. With respect to the bipolar derivation with two electrical contacts and a reference electrode, the potential difference between the two electrical contacts or between the two electrical contacts and the reference electrode is measured.
The present invention relates to a monopolar derivation of bioelectrical signals.
The electrical contact which is arranged in the electrically active area of the biological organism in the following is referred to as derivation contact or derivation electrode. The electrical contact which provides a reference signal or a reference potential in the following is referred to as reference contact or reference electrode.
The monopolar derivation known from prior art, however, is disadvantageous because the arrangement of the derivation electrode and reference electrode with respect to each other should meet two requirements which, however, are conflicting to a large extent.
According to a first requirement, the derivation electrode and the reference electrode should be placed as close as possible with respect to each other.
A distance as close as possible or a minimal distance between the derivation electrode and the reference electrode is accompanied by low electrical impedance between the two electrodes because lower volumes of biological tissue between the two contacts represent a lower electrical resistance than large volumes. Proportionally to the electrical resistance between the two electrodes, the amplitude of a thermal noise emerges which compromises or affects the quality of the bioelectrical signal to be derived or to be detected negatively.
Moreover, a smaller distance between derivation electrode and reference electrode reduces the surface of a virtual conductor loop which is created by the electrode feed lines and the current path through the biological tissue (between the two electrode contacts). The larger the surface of such a conductor loop is, the larger the electrical voltage between the derivation electrode and the reference electrode will be which is created by inductance when an alternating magnetic field passes through the conductor loop. This electrical voltage superposes the bioelectrical signal to be detected, and in the worst case, it may completely mask or bring the electronical amplifier into the saturation region preventing a possibly necessary amplification of the weak bioelectrical signal. Sources for an alternating magnetic field, for example, may be a mains current of 50 Hz, an anti-theft system in department stores, radio frequency identification systems (RFID), or the like.
According to a second requirement, the derivation electrode and the reference electrode should be placed as far as possible away from each other.
By a large geometric distance between the derivation electrode and the reference electrode it may be guaranteed that the electrical potential of the reference electrode is independent of the source of the bioelectrical signal in the vicinity of the derivation electrode. Thereby, the potential difference between the derivation electrode and the reference electrode can be maximized such that the bioelectrical signal can be detected and evaluated better, and can be distinguished more easily from interferences.
With respect to a reference electrode which is arranged close to the derivation electrode, the characteristics of the signal derivation corresponds to a bipolar derivation such that this approach leads to a mixing of the bioelectrical signals which are derived from the two contacts, and in the worst case, it prevents a detection of a bioelectrical event. Because, if both contacts (reference electrode and derivation electrode) simultaneously detect an identical or nearly identical signal, the electrical potential difference to be amplified is zero or nearly zero.
Therefore, in prior art the reference electrode is placed at some distance away from the derivation electrode in order to achieve a good signal quality and a good local selectivity which, however, requires that the surroundings is specifically interference free which in most cases is only possible under laboratory conditions.
Therefore, it is an object of the present invention to provide solutions for derivation or for detection of bioelectrical signals, in particular, bioelectrical signals of a brain which at least partially avoid the disadvantages from prior art, and which allow for a detection of bioelectrical signals independently of the arrangement of the electrodes with respect to each other.